This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Application No. 2000-061967 filed on Mar. 7, 2000, the entire content of which is incorporated herein by reference.
The present invention generally relates to vehicle brake systems. More particularly, the present invention pertains to a vehicle hydraulic brake pressure controller and a method for increasing hydraulic pressure in wheel brake cylinders of a vehicle hydraulic brake pressure controller.
Known vehicle hydraulic brake pressure controllers provided with an anti-lock braking system (ABS controller) control the braking force to maximize the friction coefficient between the road surface and wheels by decreasing, increasing, and holding the brake pressure in the wheel brake cylinder of each wheel to avoid lock up of the wheels upon sudden braking (when the brake pedal is suddenly applied). In these known anti-lock braking systems, the lock-up condition of the wheels is detected by assessing whether the wheel acceleration or wheel deceleration is greater than a predetermined threshold value or whether a slip ratio obtained based on the wheel speed and the vehicle speed is greater than a predetermined threshold value. This assessment is then used to start the anti-lock braking system control.
Typically, when the brake hydraulic pressure (Pwc) for each wheel cylinder is increased under anti-lock braking control, a pulse pressure increase time t0 is adjusted by the amount of the master cylinder hydraulic pressure (Pmc) actuated by applying the brake pedal. In other words, in the known anti-lock braking system, a stable ABS control performance is ensured by providing a pressure increase amount in accordance with the change of the stepping force of the brake pedal by adjusting the pulse pressure increase time t0 depending on the amount of the master cylinder hydraulic pressure (Pmc).
However, the known method for effecting pressure increase includes at least the following disadvantages. The master cylinder hydraulic pressure (Pmc) is proportional to the brake pedal stepping force. The brake hydraulic pressure (Pwc) for the wheel cylinder of each wheel varies depending on the friction force (that is the friction coefficient xcexc of the road surface) affecting each wheel. The friction force (the friction coefficient xcexc of road surface) is directly proportional to the brake hydraulic pressure (Pwc) for the wheel cylinder of each wheel.
The pressure increase speed (pressure increase gradient) of the brake hydraulic pressure in the wheel cylinder is determined based on the pressure differential between the master cylinder hydraulic pressure (Pmc) generated in the master cylinder and the brake hydraulic pressure (Pwc) delivered to the wheel cylinder of each wheel (Pmc-Pwc=xcex94P). Accordingly, the pressure differential (Pmc-Pwc=xcex94P) varies depending on the friction force (the friction coefficient xcexc of the road surface) affecting each wheel to change the pressure increase gradient. The pressure increase gradient is reduced at high friction coefficient xcexc of road surface due to high Pwc pressure and is increased at low friction coefficient xcexc of road surface due to low Pwc pressure.
As a result, when the pulse pressure increase is conducted with the pulse pressure increase time based on the master cylinder hydraulic pressure, the increase amount (xcex94Pxc2x7t0) may be excessively small in connection with the road surfaces having a high friction coefficient xcexc and may be too large in connection with road surfaces having a low friction coefficient xcexc.
These drawbacks make it difficult to obtain ABS control performance that is suitable for the stepping force applied to the brake pedal.
A need thus exists for an improved hydraulic brake pressure controller that obviates drawbacks associated with known hydraulic brake pressure controllers such as those described above.
A need also exists for a hydraulic brake pressure controller and method for increasing the pressure in the wheel cylinders of a hydraulic brake pressure controller that are capable of obtaining ABS control performance suitable for the condition of a driving road surface and the stepping force of the brake pedal.
One aspect of the invention involves a method for pressure increase in a wheel cylinder of a hydraulic brake pressure controller for providing a braking force to a wheel by supplying brake pressure from a hydraulic pressure generator to the wheel cylinder. The method for pressure increase in the hydraulic brake pressure controller is conducted based on the pressure differential between the hydraulic pressure of brake fluid supplied from the hydraulic pressure generator and the brake hydraulic pressure in the wheel cylinder when the brake pressure in the wheel cylinder is increased by the hydraulic pressure controller.
Compared to known systems that control the pressure increase based on only the hydraulic pressure of the brake fluid supplied from the hydraulic pressure generator, the present invention inhibits or prevents an insufficiency in the amount of the increase of the brake hydraulic pressure to the wheel cylinder under a high xcexc road surface condition while also inhibiting or preventing an excessive increase in the amount of the brake hydraulic pressure in the wheel cylinder at low xcexc road surface condition when pressurizing each wheel cylinder by the hydraulic pressure controller. As a result, ABS control performance suitable for the vehicle driving road surface and the stepping force on he brake pedal can be obtained.
According to another aspect of the invention, a method for pressure increase in wheel cylinders of a hydraulic brake pressure controller to provide a braking force to vehicle wheel by supplying brake pressure from a hydraulic pressure generator to the wheel cylinders includes determining the hydraulic brake pressure in the wheel cylinders, determining the brake pressure supplied from the hydraulic pressure generator, performing anti-lock braking control by selecting one of a pressure decreasing mode, pulse pressure increasing mode and a pressure maintaining mode, determining the pressure differential between the brake pressure supplied from the hydraulic pressure generator and the hydraulic brake pressure in the wheel cylinders, and increasing the hydraulic brake pressure in the wheel cylinders based on the determined pressure differential when the pulse pressure increasing mode is selected.
Another aspect of the invention involves a hydraulic brake pressure controller that includes a plurality of wheel cylinders each operatively associated with a respective vehicle wheel for providing a braking force to the vehicle wheel, a hydraulic pressure generator for supplying brake hydraulic pressure to the wheel cylinders, a hydraulic pressure detector for detecting hydraulic pressure supplied by the hydraulic pressure generator, a hydraulic pressure controller provided between the hydraulic pressure generator and the wheel cylinders for controlling brake hydraulic pressure in the wheel cylinders, a wheel speed detector for detecting a wheel speed of each wheel, and a wheel speed calculator for calculating the wheel speed using an output signal from the wheel speed detector. A wheel acceleration calculator calculates a wheel acceleration of each wheel using the wheel speed, an estimated vehicle deceleration calculator calculates vehicle deceleration using an output signal from the hydraulic pressure detector, and an estimated vehicle speed calculator calculates an estimated vehicle speed based on the wheel speed and the vehicle deceleration. A braking force controller controls the braking force applied to the wheels by actuating the hydraulic pressure controller in accordance with the calculated wheel speed, the calculated wheel acceleration, and the calculated estimated vehicle speed. Anti-lock braking control is actuated by the braking force controller to select any one of a pressure decrease mode, a pulse pressure increase mode, and a pressure maintaining mode by judging whether a wheel acceleration is greater than a predetermined threshold value and whether a slip ratio obtained based on the wheel speed and the estimated vehicle speed is greater than a predetermined threshold value by the braking force controller. A pulse pressure increase time of the brake hydraulic pressure used during the pulse pressure increase mode is adjusted based on a pressure differential between the pressure supplied by the hydraulic pressure generator and the brake hydraulic pressure in the wheel cylinders.
The anti-lock braking control is conducted by directing a pressure decrease mode, a pulse pressure increase mode, or a maintenance mode after judging whether the wheel acceleration is greater than a predetermined threshold value and a slip ratio obtained based on the wheel speed and the estimated vehicle speed is greater than a predetermined threshold value by the braking force controller. A pulse pressure increase time of the brake hydraulic pressure in the wheel cylinder is adjusted based on the pressure differential between the generated hydraulic pressure of the hydraulic pressure generator and the brake hydraulic pressure in the wheel cylinder.
The product of the increase amount of the brake hydraulic pressure in the wheel cylinder (i.e., the pressure differential between the generated hydraulic pressure of the hydraulic pressure generator and the brake hydraulic pressure in the wheel cylinder) and the pulse pressure increase time can be suitably adjusted for the vehicle driving road surface and the stepping force of the brake pedal. Consequently, an insufficiency in the amount of increase of brake hydraulic pressure in the wheel cylinder at high xcexc road surface condition and an excess in the amount of increase of the brake hydraulic pressure in the wheel cylinder at low xcexc road surface condition can be prevented to obtain a highly desirable and substantially optimum ABS control performance for the vehicle driving road surface and the stepping force of the brake pedal.
In the hydraulic brake pressure controller of the present invention, the brake hydraulic pressure in the wheel cylinder is calculated based on the estimated vehicle deceleration obtained from the estimated vehicle speed. Because the brake hydraulic pressure in the wheel cylinder is calculated based on the estimated vehicle deceleration obtained from the estimated vehicle speed, it is not necessary to provide detecting means for directly detecting the brake hydraulic pressure in the wheel cylinder such as a detecting sensor. This advantageously reduces the manufacturing cost of the hydraulic brake pressure controller.
In addition, in the hydraulic brake pressure controller of the present invention, a first estimated vehicle speed is calculated based on the wheel speed, and a second estimated vehicle speed is determined by subtracting a product of the estimated vehicle deceleration and a time associated with one cycle of the program or routine from the calculated value of the estimated vehicle speed of last transaction cycle. When the first estimated vehicle speed is greater than the second estimated vehicle speed, the first estimated vehicle speed is determined as the estimated vehicle speed. When the first estimated vehicle speed is equal to or less than the second estimated vehicle speed, the second estimated vehicle speed is determined as the estimated vehicle speed. Thus, the underestimation of the estimated vehicle speed due to the interference of each of the wheels of the 4WD vehicle can be avoided. The underestimation of the slip ratio estimated from the estimated vehicle speed and the wheel speed can also be avoided. Also, the delay in starting anti-lock braking control due to the misjudgment that the vehicle speed and the wheel speed are different as judged from the decline of the estimated vehicle speed accompanied by the simultaneous decline of the wheel speed of the four wheels can be inhibited or prevented.
According to a further aspect of the invention, a hydraulic brake pressure controller includes a plurality of wheel cylinders each operatively associated with a respective vehicle wheel for providing a braking force to the vehicle wheel, a hydraulic pressure generator for supplying brake hydraulic pressure to the wheel cylinders, a hydraulic pressure detector for detecting hydraulic pressure supplied by the hydraulic pressure generator, a hydraulic pressure controller provided between the hydraulic pressure generator and the wheel cylinders for controlling brake hydraulic pressure in the wheel cylinders, a braking force controller for controlling a braking force applied to the wheels to perform anti-lock braking under any one of a pressure decrease mode, a pulse pressure increase mode, and a pressure maintaining mode, and a device for determining a pulse pressure increase time for carrying out the pulse pressure increase mode based on a pressure differential between the hydraulic pressure supplied by the hydraulic pressure generator and the hydraulic brake pressure in the wheel cylinders.